In the Senate trial, Trump wants witnesses. The House managers (supported by Schumer) want witnesses. Trials usually have witnesses. So let’s have witnesses.
Or to put it another way: The prosecution wants to call some witnesses. The defense wants to call some witnesses. Let's hear the witnesses.
It may be that the defenses' witnesses will be irrelevant to the charges, and may turn part of the trial into a bit of a spectacle. But the defense can make that decision. If it backfires, and if a Senate at once well-informed and disgusted by Trump votes to convict...so be it.
